Science Musings Blog

Thursday, June 17, 2004

Wow! -- Part 3

The Cassini space craft has sent back photographs showing boulders on Saturn's moon Phoebe!

These days Saturn is lost in the Sun's light, setting soon after the Sun. But it's out there, on the other side of the Sun from Earth and ten times farther from the Sun than Earth.

Almost every school room has a solar system poster with the planets lined up like soldiers on parade, or a dangling mobile with the planets jostling each other in the breeze. These illustrations do little to help a student comprehend the scale of the solar system or the emptiness of space.

To get a better idea of Cassini's feat, imagine the Sun -- our blazing star, nearly a million miles in diameter -- as a smallish grapefruit on the goal line of a football field. On this scale, Earth is a salt grain on the 10 yard line, and Saturn is a pea at the far end of the field.

In our football-field sized solar system, the Cassini spacecraft is infinitesimally small. (In real life, it is about the size and weight of a school bus.) It was not possible with existing rocketry to make Cassini move fast enough to travel directly to Saturn, at least not without taking decades to get there. So NASA engineers used a little orbital magic. Instead of directing the craft toward Saturn at launch in October 1997, they aimed for Venus -- in the opposite direction! The craft darted around Venus in April 1998 and got a gravitational boost of energy at the expense of that planet, like a stone whirled in a sling. It coasted out past the orbit of Earth, then fell towards Venus again, where it got another kick. By this time the spacecraft had made nearly two orbits of the Sun. It climbed again, away from the Sun, meeting Earth on the 10-yard line for another gravitational boost, then to Jupiter out there on the 50-yard line for one more increment of energy.

And on to Saturn.

The photos of Phoebe are spectacular, but equally spectacular is the finesse with which navigational engineers take advantage of gravitational assists to speed their craft on their way, looping and darting like swallows through the vast emptiness of the solar system. In the case of Cassini, the early flybys of Venus and the Earth provided the equivalent of 75 tons of fuel. After the initial launch towards Venus, only small adjustments to the trajectory were necessary to direct the craft towards its next rendezvous!